Policymakers, researchers and the public have called for interagency collaboration - service providers' mutual referrals, information sharing, service evaluation and outcome dissemination. Interagency collaboration improves communication between counselors, program workers, educators, and service coordinators, thus improving integration of services and overall cost effectiveness. To advance HIV-prevention practice, we must examine interagency collaboration impact on providers' delivery of CDC's Diffusion of Effective Behavioral Interventions (DEBIs). Cross-sectional research shows negative associations between providers' perceived lack of agency resources and DEBI implementation, but positive associations between interagency collaboration and implementation. Cross-sectional data limit our understanding of temporal associations between these variables. The study of interagency collaboration has been neglected as a strategy to improve DEBI implementation. We propose the first longitudinal, mixed method study to examine the impact of interagency collaboration on providers' delivery of DEBIs. This study is grounded in diffusion of innovations, institutional and behavioral theories. Providers (n=350) recruited from 30 agencies will provide baseline before receiving training and Web-based education about interagency collaboration. A 12-month follow-up will capture immediate changes in providers' intentions to deliver DEBIs, and a 30-month follow-up will capture changes in DEBI implementation. The study will identify predictors of interagency collaboration changes and examine associations between these changes and providers' intentions and the number of DEBIs they implement over time. Qualitative data will be collected after both follow-ups to establish additional barriers to implementation and how providers overcome these barriers. This study has significant public health impact by informing future structural interventions to promote interagency collaboration. Knowing how interagency collaboration influences DEBI implementation, policy makers can intervene across HIV-prevention delivery systems. The knowledge produced by studying DEBI implementation further informs implementation of other evidence-based methods - e.g., HIV rapid testing - shown to slow HIV spread. Improving DEBI implementation will increase the proportion of consumers receiving science-based HIV prevention.